Photocurable ink jet recording ink composition and ink jet recording method

ABSTRACT

Provided is a photocurable ink jet recording ink composition having a low viscosity and excellent curing property and photopolymerization initiator solubility. The photocurable ink jet recording ink composition includes a polymerizable compound and a photopolymerization initiator. The polymerizable compound includes vinyl ether group-containing (meth)acrylic esters represented by Formula (I) in an amount of 20% by mass or more based on the total mass of the ink composition. The photopolymerization initiator includes a bisacylphosphine oxide photopolymerization initiator in an amount of 5% by mass or more based on the total mass of the ink composition and a thioxanthone photopolymerization initiator. The content ratio of the bisacylphosphine oxide photopolymerization initiator to the thioxanthone photopolymerization initiator is from 3:1 to 5:4.

This application is a divisional of and claims priority to U.S. patentapplication No. Ser. 13/611,095 filed Sep. 12, 2015, and claims priorityunder 35 U.S.C. §119 to Japanese Patent Application No. 2011-198184filed Sep. 12, 2015, each of which are hereby incorporated by referencein its entirety.

BACKGROUND

1. Technical Field

The present invention relates to a photocurable ink jet recording inkcomposition and an ink jet recording method.

2. Related Art

Various systems have been utilized as recording methods of formingimages on recording media such as paper based on image data signals.Among them, an ink jet system is an inexpensive apparatus and directlyforms an image on a recording medium by discharging inks on a necessaryimage area only and therefore efficiently uses inks at low running cost.In addition, the ink jet system is low in noise and is thereforeexcellent as a recording method.

Recently, in the recording method of the ink jet system, a photocurableink composition, which is cured by light irradiation, is used as an inkcomposition that can provide high water resistance, solvent resistance,abrasion resistance, etc.

For example, JP-A-11-302562 discloses a photocurable paint compositionthat contains 0.1 to 20 parts by weight of a thioxanthonephotopolymerization initiator and 1 to 20 parts by weight of anacylphosphine or benzophenone photoradical generator based on 100 partsby weight of a coating film-forming binder component composed of 20 to40% by weight of an acryloyl group-containing polyester oligomer havinga number-average molecular weight of 2400 to 3150 or bisphenol Aepoxyacrylate, 20 to 40% by weight of N-acryloylmorpholine, and 10% byweight of pentaerythritol triacrylate or dipentaerythritol hexaacrylate.

Unfortunately, when the photocurable paint composition described inJP-A-11-302562 is applied to ink jet recording, the composition causesproblems, i.e., deterioration in discharge stability, deterioration instorage stability due to poor solubility of the photopolymerizationinitiator, and deterioration in curing property.

SUMMARY

An advantage of some aspects of the invention is to provide aphotocurable ink jet recording ink composition having excellent curingproperty and photopolymerization initiator solubility.

In general, a photocurable ink composition contains a polymerizablecompound and a photopolymerization initiator. First, the inventors haveinvestigated polymerizable compounds from the viewpoint of obtaining anink having a low viscosity and an excellent curing property and havefound that use of a predetermined amount of certain vinyl ethergroup-containing (meth)acrylic esters is effective. Secondly, theinventors have investigated photopolymerization initiators from theviewpoint of obtaining an ink having an excellent curing property andhave found that use of a predetermined amount of a bisacylphosphineoxide photopolymerization initiator is effective. In inks merelycontaining these components, however, the solubility of thebisacylphosphine oxide photopolymerization initiator is low, and thecuring property is not enhanced to a practical level.

The inventors have conducted further investigation and have found thatthe above-mentioned problems can be solved by adding, to a photocurableink jet recording ink composition containing the bisacylphosphine oxidephotopolymerization initiator, a thioxanthone photopolymerizationinitiator in an amount of a predetermined content ratio range with thebisacylphosphine oxide photopolymerization initiator. The compositionhas excellent curing property and photopolymerization initiatorsolubility. Thus, the invention has been accomplished.

That is, the invention includes the following aspects.

[1] A photocurable ink jet recording ink composition including apolymerizable compound and a photopolymerization initiator, wherein thepolymerizable compound includes: vinyl ether group-containing(meth)acrylic esters represented by the following Formula (I):CH₂═CR¹—COOR²—O—CH═CH—R³   (I)(wherein, R¹ represents a hydrogen atom or a methyl group; R² representsa divalent organic residue having 2 to 20 carbon atoms; and R³represents a hydrogen atom or a monovalent organic residue having 1 to11 carbon atoms) in an amount of 20% by mass or more based on the totalmass of the ink composition; and the photopolymerization initiatorincludes a bisacylphosphine oxide photopolymerization initiator in anamount of 5% by mass or more based on the total mass of the inkcomposition and a thioxanthone photopolymerization initiator, whereinthe content ratio of the bisacylphosphine oxide photopolymerizationinitiator to the thioxanthone photopolymerization initiator is from 3:1to 5:4 by mass standard.

[2] The photocurable ink jet recording ink composition according toaspect [1], wherein the thioxanthone photopolymerization initiator is atleast one of 2,4-diethylthioxanthone and 2-isopropylthioxanthone.

[3] The photocurable ink jet recording ink composition according toaspect [1] or [2], wherein the content of the thioxanthonephotopolymerization initiator is 4% by mass or less based on the totalmass of the ink composition.

[4] The photocurable ink jet recording ink composition according to anyone of aspects [1] to [3], wherein the bisacylphosphine oxidephotopolymerization initiator isbis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide.

[5] The photocurable ink jet recording ink composition according to anyone of aspects [1] to [4], wherein the polymerizable compound furtherincludes phenoxyethyl (meth)acrylate in an amount of less than 30% bymass based on the total mass of the ink composition.

[6] The photocurable ink jet recording ink composition according to anyone of aspects [1] to [5], wherein the vinyl ether group-containing(meth)acrylic esters are 2-(vinyloxyethoxy)ethyl acrylate.

[7] An ink jet recording method including: discharging a photocurableink jet recording ink composition according to any one of aspects [1] to[6] onto a recording medium; and curing the discharged ink compositionby irradiating the ink composition with ultraviolet light having anemission peak wavelength in a range of 350 to 420 nm using an LED.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments according to the invention will now be described in detail.The invention is not limited to the following embodiments, and variousmodifications can be made within the scope of the invention.

Throughout the specification, the term “(meth)acrylate” refers toacrylate and/or methacrylate corresponding to the acrylate, and“(meth)acrylic” refers to acrylic and/or methacrylic corresponding tothe acrylic.

Throughout the specification, the term “curing property” refers to aproperty of curing upon exposure to light. The term “color stability”refers to a property that the L*a*b* in the CIE Lab (L*a*b* colorsystem) immediately after printing hardly changes with passage of time.The term “discharge stability” refers to a property that discharge ofink droplets from a nozzle is constantly stable without clogging of thenozzle. The term of “storage stability” refers to a property that noprecipitate is observed when an ink is stored at −20° C. for four daysor a property that no increase in viscosity is observed when an ink isstored at 60° C. for seven days.

Photocurable Ink Jet Recording Ink Composition

The photocurable ink jet recording ink composition (hereinafter, alsosimply referred to as “ink composition”) according to an embodiment ofthe invention includes a polymerizable compound and aphotopolymerization initiator. The polymerizable compound includes vinylether group-containing (meth)acrylic esters represented by the followingFormula (I):CH₂═CR¹—COOR²—O—CH═CH—R³   (I)(wherein, R¹ represents a hydrogen atom or a methyl group; R² representsa divalent organic residue having 2 to 20 carbon atoms; and R³represents a hydrogen atom or a monovalent organic residue having 1 to11 carbon atoms) in an amount of 20% by mass or more based on the totalmass of the ink composition. The photopolymerization initiator includesa bisacylphosphine oxide photopolymerization initiator in an amount of5% by mass or more based on the total mass of the ink composition and athioxanthone photopolymerization initiator. The content ratio of thebisacylphosphine oxide photopolymerization initiator to the thioxanthonephotopolymerization initiator (the former:the latter) is from 3:1 to 5:4by mass standard.

Additives (components) that are contained or optionally contained in theink composition of the embodiment will now be described.

Polymerizable Compounds

The polymerizable compound contained in the ink composition of theembodiment polymerizes by the effect of the photopolymerizationinitiator described below when irradiated with ultraviolet light toharden the printed ink. Vinyl ether group-containing (meth)acrylicesters

The vinyl ether group-containing (meth)acrylic esters, which are thepolymerizable compound indispensable in the embodiment, are representedby Formula (I).

An ink composition containing the vinyl ether group-containing(meth)acrylic esters can decrease the viscosity of an ink to make thecuring property of the ink satisfactory.

In Formula (I), the divalent organic residue having 2 to 20 carbon atomsrepresented by R² is preferably a linear, branched, or circular alkylenegroup having 2 to 20 carbon atoms; an alkylene group having 2 to 20carbon atoms and having an oxygen atom by an ether bond and/or an esterbond in its structure; or an optionally substituted divalent aromaticgroup having 6 to 11 carbon atoms. Among them, more preferred arealkylene groups having 2 to 6 carbon atoms, such as ethylene,n-propylene, isopropylene, and butylene groups; and alkylene groupshaving 2 to 9 carbon atoms and having an oxygen atom by an ether bond inits structure, such as oxyethylene, oxy-n-propylene, oxyisopropylene,and oxybutylene groups.

In Formula (I), the monovalent organic residue having 1 to 11 carbonatoms represented by R³ is preferably a linear, branched, or circularalkyl group having 1 to 10 carbon atoms or an optionally substitutedaromatic group having 6 to 11 carbon atoms. Among them, more preferredare alkyl groups having one or two carbon atoms, such as methyl andethyl groups; and aromatic groups having 6 to 8 carbon atoms, such asphenyl and benzyl groups.

When the organic residues are groups optionally having substituents, thesubstituents are classified into groups containing carbon atoms andgroups not containing carbon atoms. In the case of a substituentcontaining a carbon atom, the carbon atom is counted as the carbon atomof the organic residue. Examples of the group containing a carbon atominclude, but not limited to, carboxyl and alkoxy groups. Examples of thegroup not containing a carbon atom include, but not limited to, hydroxyland halo groups.

Examples of the vinyl ether group-containing (meth)acrylic estersinclude, but not limited to, 2-vinyloxyethyl (meth) acrylate,3-vinyloxypropyl (meth)acrylate, 1-methyl-2-vinyloxyethyl(meth)acrylate, 2-vinyloxypropyl (meth) acrylate, 4-vinyloxybutyl (meth)acrylate, 1-methyl-3-vinyloxypropyl (meth) acrylate,1-vinyloxymethylpropyl (meth) acrylate, 2-methyl-3-vinyloxypropyl (meth)acrylate, 1,1-dimethyl-2-vinyloxyethyl (meth) acrylate, 3-vinyloxybutyl(meth) acrylate, 1-methyl-2-vinyloxypropyl (meth) acrylate,2-vinyloxybutyl (meth) acrylate, 4-vinyloxycyclohexyl (meth) acrylate,6-vinyloxyhexyl (meth)acrylate, 4-vinyloxymethylcyclohexylmethyl (meth)acrylate, 3-vinyloxymethylcyclohexylmethyl (meth) acrylate,2-vinyloxymethylcyclohexylmethyl (meth) acrylate,p-vinyloxymethylphenylmethyl (meth) acrylate,m-vinyloxymethylphenylmethyl (meth) acrylate,o-vinyloxymethylphenylmethyl (meth) acrylate, 2-(vinyloxyethoxy)ethyl(meth) acrylate, 2-(vinyloxyisopropoxy)ethyl (meth) acrylate,2-(vinyloxyethoxy)propyl (meth) acrylate, 2-(vinyloxyethoxy)isopropyl(meth) acrylate, 2-(vinyloxyisopropoxy)propyl (meth) acrylate,2-(vinyloxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyethoxy)ethyl (meth) acrylate,2-(vinyloxyethoxyisopropoxy)ethyl (meth) acrylate,2-(vinyloxyisopropoxyethoxy)ethyl (meth) acrylate,2-(vinyloxyisopropoxyisopropoxy)ethyl (meth)acrylate,2-(vinyloxyethoxyethoxy)propyl (meth) acrylate,2-(vinyloxyethoxyisopropoxy)propyl (meth) acrylate,2-(vinyloxyisopropoxyethoxy)propyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)propyl (meth)acrylate,2-(vinyloxyethoxyethoxy)isopropyl (meth) acrylate,2-(vinyloxyethoxyisopropoxy)isopropyl (meth) acrylate,2-(vinyloxyisopropoxyethoxy)isopropyl (meth)acrylate,2-(vinyloxyisopropoxyisopropoxy)isopropyl (meth)acrylate,2-(vinyloxyethoxyethoxyethoxy)ethyl (meth) acrylate,2-(vinyloxyethoxyethoxyethoxyethoxy)ethyl (meth) acrylate,2-(isopropenoxyethoxy)ethyl (meth) acrylate,2-(isopropenoxyethoxyethoxy)ethyl (meth) acrylate,2-(isopropenoxyethoxyethoxyethoxy)ethyl (meth) acrylate,2-(isopropenoxyethoxyethoxyethoxyethoxy)ethyl (meth) acrylate,polyethylene glycol monovinyl ether (meth)acrylate, and polypropyleneglycol monovinyl ether (meth)acrylate.

Among them, 2-(vinyloxyethoxy)ethyl (meth)acrylate, i.e.,2-(vinyloxyethoxy)ethyl acrylate and 2-(vinyloxyethoxy)ethylmethacrylate can notably decrease the viscosity of an ink, have highflash points, and can provide an excellent curing property to an ink andare therefore preferred. 2-(Vinyloxyethoxy)ethyl acrylate is morepreferred. 2-(Vinyloxyethoxy)ethyl (meth)acrylate includes2-(2-vinyloxyethoxy)ethyl (meth)acrylate and 2-(1-vinyloxyethoxy)ethyl(meth)acrylate. 2-(Vinyloxyethoxy)ethyl acrylate includes2-(2-vinyloxyethoxy)ethyl acrylate and 2-(1-vinyloxyethoxy)ethylacrylate. 2-(Vinyloxyethoxy)ethyl acrylate is superior to2-(vinyloxyethoxy)ethyl methacrylate in the light of curing property.

The content of the vinyl ether group-containing (meth)acrylic esters ispreferably 20% by mass or more, more preferably 20 to 80% by mass, morepreferably 20 to 50% by mass, more preferably 20 to 40% by mass, andmost preferably 20 to 30% by mass, based on the total mass (100% bymass) of the ink composition. Within the content range above, theviscosity of the ink composition can be maintained low, and also the inkcomposition can have an excellent curing property. In view of alsomaintaining the viscosity of the ink composition to be further low, thecontent is preferably 50 to 80% by mass, more preferably 50 to 70% bymass, and most preferably 60 to 70% by mass.

Examples of the method of producing the vinyl ether group-containing(meth)acrylic esters include, but not limited to, esterification of(meth)acrylic acid and a hydroxyl group-containing vinyl ether (processB); esterification of a (meth)acrylic acid halide and a hydroxylgroup-containing vinyl ether (process C); esterification of a(meth)acrylic anhydride and a hydroxyl group-containing vinyl ether(process D); ester exchange of a (meth)acrylic ester and a hydroxylgroup-containing vinyl ether (process E); esterification of(meth)acrylic acid and a halogen-containing vinyl ether (process F);esterification of an alkaline (earth) metal salt of (meth)acrylic acidand a halogen-containing vinyl ether (process G); vinyl exchange of ahydroxyl group-containing (meth)acrylic ester and a vinyl carboxylate(process H); and ether exchange of a hydroxyl group-containing(meth)acrylic ester and an alkyl vinyl ether (process I).

Among these methods, process E can further exhibit the effects desiredin the embodiment and is therefore preferred.

Polymerizable Compound other than Vinyl Ether Group-Containing(Meth)Acrylic Esters

In addition to the vinyl ether group-containing (meth)acrylic esters,the ink composition may further contain various known monomers andoligomers, such as monofunctional, difunctional, and tri- ormore-functional monomers and oligomers (hereinafter, referred to as“other polymerizable compound”). Examples of the monomer includeunsaturated carboxylic acids (e.g., (meth)acrylic acid, itaconic acid,crotonic acid, isocrotonic acid, and maleic acid) and salts and estersthereof, urethanes, amides and anhydrides thereof, acrylonitrile,styrene, various unsaturated polyesters, unsaturated polyethers,unsaturated polyamides, and unsaturated urethanes. Examples of theoligomer include oligomers formed from the monomers mentioned above(e.g., linear acrylic oligomers), epoxy (meth)acrylate, oxetane(meth)acrylate, aliphatic urethane (meth)acrylate, aromatic urethane(meth)acrylate, and polyester (meth) acrylate.

As the other monofunctional monomer or multifunctional monomer, anN-vinyl compound may be contained in the ink composition. Examples ofthe N-vinyl compound include N-vinyl formamide, N-vinyl carbazole,N-vinyl acetamide, N-vinyl pyrrolidone, N-vinylcaprolactam, andacryloylmorpholine, and derivatives thereof.

Among other polymerizable compounds, ester of (meth)acrylic acid, i.e.,(meth)acrylate is preferable.

In the (meth)acrylates mentioned above, examples of the monofunctional(meth)acrylate include isoamyl (meth)acrylate, stearyl (meth)acrylate,lauryl (meth)acrylate, octyl (meth)acrylate, decyl (meth)acrylate,isomyristyl (meth)acrylate, isostearyl (meth)acrylate,2-ethylhexyl-diglycol (meth) acrylate, 2-hydroxybutyl (meth) acrylate,butoxyethyl (meth) acrylate, ethoxydiethylene glycol (meth)acrylate,methoxydiethylene glycol (meth) acrylate, methoxypolyethylene glycol(meth) acrylate, methoxypropylene glycol (meth) acrylate, phenoxyethyl(meth)acrylate, tetrahydrofurfuryl (meth)acrylate, isobornyl(meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, lactone-modifiedflexible (meth)acrylate, t-butylcyclohexyl (meth) acrylate,dicyclopentanyl (meth) acrylate, and dicyclopentenyloxyethyl (meth)acrylate. Among these monofunctional (meth)acrylates, phenoxyethyl(meth)acrylate exhibits good compatibility with additives and istherefore preferred, in particular, phenoxyethyl acrylate is preferred.

In the (meth)acrylates mentioned above, examples of the difunctional(meth)acrylate include triethylene di(meth)acrylate, tetraethyleneglycol di(meth)acrylate, polyethylene glycol di(meth)acrylate,dipropylene glycol di(meth)acrylate, tripropylene glycoldi(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanedioldi(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,9-nonanedioldi(meth)acrylate, neopentyl glycol di(meth)acrylate,dimethylol-tricyclodecane di(meth)acrylate, EO (ethylene oxide) adductdi(meth)acrylate of bisphenol A, PO (propylene oxide) adductdi(meth)acrylate of bisphenol A, hydroxypivalic acid neopentyl glycoldi(meth)acrylate, and polytetramethylene glycol di(meth)acrylate. Amongthese difunctional (meth)acrylates, dipropylene glycol di(meth)acrylatecan provide a tough coating film and a low viscosity and is thereforepreferred.

In the (meth)acrylates mentioned above, examples of the tri- ormore-functional (meth)acrylate include trimethylolpropanetri(meth)acrylate, EO-modified trimethylolpropane tri(meth)acrylate,pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, ditrimethylolpropanetetra(meth)acrylate, glycerin propoxy tri(meth)acrylate,caprolactone-modified trimethylolpropane tri(meth)acrylate,pentaerythritolethoxy tetra(meth)acrylate, and caprolactam-modifieddipentaerythritol hexa(meth)acrylate. Among these multifunctional (meth)acrylates, trimethylolpropane tri(meth)acrylate can provide a toughcoating film and a low viscosity and is therefore preferred.

Among these compounds, the other polymerizable compound preferablyincludes a monofunctional (meth)acrylate. In such a case, the inkcomposition can have a low viscosity and excellent storage stability,and excellent discharge stability can be easily obtained in ink jetrecording. Furthermore, a combination use of a monofunctional(meth)acrylate and a difunctional (meth)acrylate increases thetoughness, heat resistance, and chemical resistance of a coating filmand is therefore preferred.

The monofunctional (meth)acrylate preferably has one or more skeletonsselected from the group consisting of aromatic ring skeletons, saturatedalicyclic skeletons, and unsaturated alicyclic skeletons. When the otherpolymerizable compound is the monofunctional (meth)acrylate having theskeleton mentioned above, the ink composition can have a low viscosity.

Examples of the monofunctional (meth)acrylate having an aromatic ringskeleton include phenoxyethyl (meth)acrylate and2-hydroxy-3-phenoxypropyl (meth) acrylate. Examples of themonofunctional (meth)acrylate having a saturated alicyclic skeletoninclude isobornyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate,and dicyclopentanyl (meth)acrylate. Examples of the monofunctional(meth)acrylate having an unsaturated alicyclic skeleton includedicyclopentenyloxyethyl (meth) acrylate.

Among these compounds, phenoxyethyl (meth)acrylate and isobornyl(meth)acrylate can reduce viscosity and odor and are thereforepreferred. In particular, phenoxyethyl (meth)acrylate is more preferred,and phenoxyethyl acrylate is further preferred.

When the ink composition of the embodiment contains a polymerizablecompound in addition to the vinyl ether group-containing (meth)acrylicesters, the content of the polymerizable compound is preferably 40% bymass or less, more preferably 20 to 40% by mass, based on the total mass(100% by mass) of the ink composition. Within the content range above,the solubility of additives is excellent, and a coating film can havehigh toughness, heat resistance, and chemical resistance.

In particular, when the ink composition of the embodiment containsphenoxyethyl (meth)acrylate, the content of the phenoxyethyl(meth)acrylate is preferably less than 30% by mass, more preferably 5%by mass or more and less than 30% by mass, more preferably 10% by massor more and less than 30% by mass, and most preferably 10 to 25% bymass, based on the total mass (100% by mass) of the ink composition.Within the content range above, the viscosity of the ink composition canbe maintained low, and also the ink composition can have an excellentcuring property.

The above-mentioned polymerizable compounds may be used alone or in acombination of two or more thereof.

Photopolymerization Initiator

The photopolymerization initiator contained in the ink composition ofthe embodiment allows an ink present on a recording medium surface tocure by photopolymerization due to ultraviolet light irradiation toachieve printing. In radiation, ultraviolet (UV) is excellent in safety,and the light source lamp thereof is inexpensive.

Bisacylphosphine Oxide Photopolymerization Initiator

The photopolymerization initiator in the embodiment indispensablycontains a bisacylphosphine oxide photopolymerization initiator. In anink composition containing the bisacylphosphine oxidephotopolymerization initiator according to the embodiment, the curingproperty and the color stability of the ink composition are enhanced inconnection with the polymerizable compound in the embodiment.

Any bisacylphosphine oxide photopolymerization initiator can be used,and examples thereof include bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphineoxide.

Among these compounds, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxidehas high reactivity to UV-LED light on the longer wavelength side and istherefore preferred.

Examples of commercially available bisacylphosphine oxidephotopolymerization initiator include IRGACURE 819(bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide), IRGACURE 1800(mixture of CGI 403:IRGACURE 184=1:3), and IRGACURE 1870 (mixture of CGI403:IRGACURE 184=7:3) (each trade name manufactured by BASF).

The compound name of CGI 403 isbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide. Thecompound name of IRGACURE 184 is 1-hydroxycyclohexyl-phenyl ketone.

The ink composition may contain, in addition to the bisacylphosphineoxide photopolymerization initiator, a monoacylphosphine oxidephotopolymerization initiator such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide, 2,4,6-triethylbenzoyl diphenylphosphine oxide,or 2,4,6-triphenylbenzoyl diphenylphosphine oxide. Though themonoacylphosphine oxide photopolymerization initiator does not highlyenhance the curing property of the ink composition compared to thebisacylphosphine oxide photopolymerization initiator, the curingproperty of the ink composition is further increased by adding themonoacylphosphine oxide photopolymerization initiator, in addition tothe bisacylphosphine oxide photopolymerization initiator, to the inkcomposition. The solubility of the monoacylphosphine oxidephotopolymerization initiator in an ink composition is higher than thatof the bisacylphosphine oxide photopolymerization initiator, and,therefore, the ink composition can contain the monoacylphosphine oxidephotopolymerization initiator without any problem.

The bisacylphosphine oxide photopolymerization initiators mentionedabove may be used alone or in a combination of two or more thereof.

The content of the bisacylphosphine oxide photopolymerization initiatoris 5% by mass or more, more preferably 5 to 10% by mass, more preferably5 to 8% by mass, and most preferably 5.75 to 6.25% by mass, based on thetotal mass (100% by mass) of the ink composition. Within the contentrange above, the ink composition can have further excellent curingproperty and color stability.

Thioxanthone Photopolymerization Initiator

The photopolymerization initiator in the embodiment indispensablycontains a thioxanthone photopolymerization initiator. In an inkcomposition containing the thioxanthone photopolymerization initiatoraccording to the embodiment, the curing property of the ink compositionis enhanced in connection with the polymerizable compound in theembodiment.

Any thioxanthone photopolymerization initiator can be used, and examplesthereof include thioxanthone, 2-methylthioxanthone,2,4-diethylthioxanthone (DETX), 2-isopropylthioxanthone (ITX),2-chlorothioxanthone, and 2,4-diethylthioxanthone.

Among these compounds, 2,4-diethylthioxanthone and2-isopropylthioxanthone have excellent reactivity to ultraviolet lightin the longer wavelength region and is therefore preferred.

Examples of commercially available thioxanthone photopolymerizationinitiator include KAYACURE DETX-S (2,4-diethylthioxanthone), KAYACUREITX (2-isopropylthioxanthone) (each trade name manufactured by NipponKayaku Co., Ltd.) and Quantacure CTX (manufactured by Aceto Chemical).

These thioxanthone photopolymerization initiators may be used alone orin a combination of two or more thereof.

The content of the thioxanthone photopolymerization initiator ispreferably 4% by mass or less, more preferably 1 to 4% by mass, morepreferably 1.5 to 4% by mass, and most preferably 2.0 to 3.5% by mass,based on the total mass (100% by mass) of the ink composition. Withinthe content range above, the ink composition can have further excellentcolor stability.

When an ink composition contains both the bisacylphosphine oxidephotopolymerization initiator and the thioxanthone photopolymerizationinitiator in a predetermined content ratio range, the solubility of eachinhibitor is higher than that when these inhibitors are contained alonein the ink composition, and also the curing property of the inkcomposition can be enhanced while maintaining a low viscosity of the inkcomposition. More specifically, the bisacylphosphine oxidephotopolymerization initiator and the thioxanthone photopolymerizationinitiator contained in an ink composition at a predetermined contentratio serve as dissolution assistants for each other. This allows eachof the initiators to be dissolved in the vinyl ether group-containing(meth)acrylic esters having a low viscosity in an amount exceeding thesolubility when they are used alone. As a result, it is possible toprovide an ink composition having a low viscosity and excellent curingproperty and storage stability and also having excellent colorstability.

The predetermined range of the content ratio between the both initiatorsfor expressing such excellent effects is from 3:1 to 5:4 by massstandard. That is, the ratio B/C of the content B of thebisacylphosphine oxide photopolymerization initiator based on the totalmass (100% by mass) of the ink composition to the content C of thethioxanthone photopolymerization initiator based on the total mass (100%by mass) of the ink composition is 1.25 to 3. If the content ratio ofthe both is out of the predetermined range, the above-described highsolubility of the photopolymerization initiators is not achieved, andthe ink composition does not have such excellent curing property,storage stability, and color stability.

Other Photopolymerization Initiator

The ink composition according to the embodiment may contain aphotopolymerization initiator other than the bisacylphosphine oxidephotopolymerization initiator and the thioxanthone photopolymerizationinitiator. Any photopolymerization initiator can be used as such anadditional photopolymerization initiator, and examples thereof includeamine compounds such as triethanolamine, methyldiethanolamine,triisopropanolamine, 4,4′-dimethylaminobenzophenone,4,4′-diethylaminobenzophenone, ethyl 2-dimethylaminobenzoate, ethyl4-dimethylaminobenzoate, isoamyl 4-dimethylaminobenzoate, n-butoxyethyl4-dimethylaminobenzoate, 2-ethylhexyl 4-dimethylaminobenzoate, andtriphenylamine.

These additional photopolymerization initiators may be used alone or ina combination of two or more thereof. The content of such additionalphotopolymerization initiators in an ink composition can be the residueof the content of other additives (components) and therefore is notparticularly restricted as long as the effects of the invention are notimpaired.

Coloring Material

The ink composition of the embodiment may further contain a coloringmaterial. The coloring material can be a pigment and/or a dye.

Pigment

In the embodiment, the use of a pigment as the coloring material canincrease the light resistance of the ink composition. The pigment may bean inorganic pigment or an organic pigment.

As the inorganic pigment, for example, carbon blacks (e.g., C.I. PigmentBlack 7), such as furnace black, lamp black, acetylene black, andchannel black; iron oxide; and titanium oxide can be used.

Examples of the organic pigment include azo pigments, such as insolubleazo pigments, condensed azo pigments, azo lake, and chelate azopigments; polycyclic pigments, such as phthalocyanine pigments, peryleneand perinone pigments, anthraquinone pigments, quinacridone pigments,dioxane pigments, thioindigo pigments, isoindolinone pigments, andquinophthalone pigments; dye chelates (e.g., basic dye chelates and aciddye chelates); dye lakes (e.g., basic dye lakes and acid dye lakes); andnitro pigments, nitroso pigments, aniline black, and daylightfluorescent pigments.

More specifically, examples of the carbon black used in black inksinclude No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7,MA8, MA100, and No. 2200B (each trade name manufactured by MitsubishiChemical Corporation); Raven 5750, Raven 5250, Raven 5000, Raven 3500,Raven 1255, and Raven 700 (each trade name manufactured by CarbonColumbia); Regal 400R, Regal 330R, Regal 660R, Mogul L, Monarch 700,Monarch 800, Monarch 880, Monarch 900, Monarch 1000, Monarch 1100,Monarch 1300, and Monarch 1400 (each trade name manufactured by CABOTJAPAN K.K.); and Color Black FW1, Color Black FW2, Color Black FW2V,Color Black FW18, Color Black FW200, Color Black S150, Color Black S160,Color Black S170, Printex 35, Printex U, Printex V, Printex 140U,Special Black 6, Special Black 5, Special Black 4A, and Special Black 4(each trade name manufactured by Degussa).

Examples of the pigment used in white inks include C.I. Pigment White 6,18, and 21.

Examples of the pigment used in yellow inks include C.I. Pigment Yellow1, 2, 3, 4, 5, 6, 7, 10, 11, 12, 13, 14, 16, 17, 24, 34, 35, 37, 53, 55,65, 73, 74, 75, 81, 83, 93, 94, 95, 97, 98, 99, 108, 109, 110, 113, 114,117, 120, 124, 128, 129, 133, 138, 139, 147, 151, 153, 154, 155, 167,172, and 180.

Examples of the pigment used in magenta inks include C.I. Pigment Red 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23,30, 31, 32, 37, 38, 40, 41, 42, 48(Ca), 48(Mn), 57(Ca), 57:1, 88, 112,114, 122, 123, 144, 146, 149, 150, 166, 168, 170, 171, 175, 176, 177,178, 179, 184, 185, 187, 202, 209, 219, 224, and 245; and C.I. PigmentViolet 19, 23, 32, 33, 36, 38, 43, and 50.

Examples of the pigment used in cyan inks include C.I. Pigment Blue 1,2, 3, 15, 15:1, 15:2, 15:3, 15:34, 15:4, 16, 18, 22, 25, 60, 65, and 66;and C.I. Vat Blue 4 and 60.

Examples of the pigment other than magenta, cyan, and yellow pigmentsinclude C.I. Pigment Green 7 and 10; C.I. Pigment Brown 3, 5, 25, and26; and C.I. Pigment Orange 1, 2, 5, 7, 13, 14, 15, 16, 24, 34, 36, 38,40, 43, and 63.

These pigments may be used alone or in a combination of two or morethereof.

In the case of containing a pigment in the ink composition, the pigmentpreferably has an average particle diameter of 2 μm or less, morepreferably 30 to 300 nm. Within the average particle diameter rangeabove, the ink composition is provided with higher reliability, such asdischarge stability, storage stability, and dispersion stability, andalso can form an image with excellent image quality. Throughout thespecification, the average particle diameter is measured by dynamiclight scattering.

Dye

In the embodiment, the coloring material may be a dye. Any dye, such asacid dyes, direct dyes, reactive dyes, and basic dyes, can be used.Examples of the dye include C.I. Acid Yellow 17, 23, 42, 44, 79, and142; C.I. Acid Red 52, 80, 82, 249, 254, and 289; C.I. Acid Blue 9, 45,and 249; C.I. Acid Black 1, 2, 24, and 94; C.I. Food Black 1 and 2; C.I.Direct Yellow 1, 12, 24, 33, 50, 55, 58, 86, 132, 142, 144, and 173;C.I. Direct Red 1, 4, 9, 80, 81, 225, and 227; C.I. Direct Blue 1, 2,15, 71, 86, 87, 98, 165, 199, and 202; C.I. Direct Black 19, 38, 51, 71,154, 168, 171, and 195; C.I. Reactive Red 14, 32, 55, 79, and 249; andC.I. Reactive Black 3, 4, and 35.

These dyes may be used alone or in a combination of two or more thereof.

The content of the coloring material is preferably 1 to 20% by massbased on the total mass (100% by mass) of the ink composition. Withinthe content range, satisfactory color-developing properties can beobtained, and prevention of curing of a coating film due to lightabsorption by the coloring material itself can be decreased.

Dispersant

When the ink composition of the embodiment contains a pigment, the inkcomposition may further contain a dispersant for well dispersing thepigment. Any dispersant can be used. For example, a dispersant that iscommonly used for preparing a pigment-dispersed liquid, such as apolymer dispersant, can be used. Specific examples of the dispersantinclude those of which main component is at least one selected frompolyoxyalkylene polyalkylene polyamines, vinyl polymers and copolymers,acrylic polymers and copolymers, polyesters, polyamides, polyimides,polyurethanes, amino polymers, silicon-containing polymers,sulfur-containing polymers, fluorine-containing polymers, and epoxyresins. Examples of commercially available polymer dispersants includeAjisper series (trade name) manufactured by Ajinomoto Fine-Techno,Solsperse series (e.g., Solsperse 36000 (trade name) available fromAvecia Co.), Disperbyk series (trade name) manufactured by BYK Chemie,and Disparlon series (trade name) manufactured by Kusumoto Chemicals.

Leveling Agent

The ink composition of the embodiment may further contain a levelingagent (surfactant), which improves wettability to a printing basematerial (recoding medium). Any leveling agent can be used. For example,silicone surfactants such as polyester-modified silicone andpolyether-modified silicone can be used. Polyether-modifiedpolydimethylsiloxane and polyester-modified polydimethylsiloxane areparticularly preferred. Specific examples of the leveling agent includeBYK-347, BYK-348, BYK-UV3500, 3510, 3530, and 3570 (each trade namemanufactured by BYK Japan K.K.).

Polymerization Inhibitor

The ink composition of the embodiment may further contain apolymerization inhibitor for improving the storage stability of the inkcomposition. Any polymerization inhibitor can be used. For example,IRGASTAB UV10 and UV22 (each trade name manufactured by BASF) andhydroquinone monomethyl ether (MEHQ: a trade name manufactured by KANTOCHEMICAL CO., Inc.) can be used.

Other Additives

The ink composition of the embodiment may contain additives (components)in addition to the above-described additives. Such components are notparticularly limited. For example, generally known penetrationaccelerator and wetting agent (moisturizing agent) and other additivescan be used. Examples of the other additives include generally knownfixing agents, antifungal agents, preservatives, antioxidants, UVabsorbers, chelating agents, pH adjusters, and thickeners.

Furthermore, as described below, the ink composition of the embodimentcan be cured by irradiation with light (ultraviolet light) having anemission peak wavelength in the range of 350 to 420 nm at an irradiationenergy of 200 mJ/cm².

Recording Medium

The ink composition of the embodiment is discharged on a recordingmedium by a recording method described below to form a recorded matter.The recording medium may be absorbable or nonabsorbable. The ink jetrecording method according to an embodiment described below can bewidely applied to various recording media exhibiting highly differentdegrees of absorbability, i.e., from non-absorbing media into whichaqueous inks hardly permeate to absorbing media into which aqueous inkseasily permeate.

Any absorbing recording medium can be used, and examples thereof includeplain paper such as electrophotographic paper into which aqueous inkseasily permeate, ink jet paper (special paper for ink jet printing andhaving an ink absorbing layer composed of silica particles or aluminaparticles or an ink absorbing layer made of a hydrophilic polymer suchas polyvinyl alcohol (PVA) or polyvinyl pyrrolidone (PVP)), and paperinto which aqueous inks relatively hardly permeate and used in ordinaryoffset printing, such as art paper, coated paper, and cast-coated paper.

Any non-absorbing recording medium can be used, and examples thereofinclude films and plates of plastics such as polyvinyl chloride (PVC),polyethylene, polypropylene, and polyethylene terephthalate (PET);plates of metals such as iron, silver, copper, and aluminum; metalplates and plastic films coated with such various metals by vapordeposition; and plates of alloys such as stainless steel and brass.

Ink Jet Recording Method

An embodiment of the invention relates to an ink jet recording method.The photocurable ink jet recording ink composition according to theembodiment described above can be used in the recording method of thisembodiment. The recording method includes a discharging step ofdischarging the above-described ink composition toward a recordingmedium; and a curing step of curing the ink composition discharged inthe discharging step by irradiating the ink composition with ultravioletlight. The ink composition thus cured on the recording medium forms acoating film (cured film).

Discharging Step

In the discharging step, an ink composition is discharged onto arecording medium such that the ink composition adheres to the recordingmedium. The ink composition at the time of being discharged preferablyhas a viscosity of less than 11 mPa·s, more preferably 4 mPa·s or moreand less than 11 mPa·s, more preferably 6 mPa·s or more and less than 11mPa·s, and most preferably 10.6 to 10.9 mPa·s. In the case of an inkcomposition having such a viscosity at room temperature or in anon-heated state, the ink composition may be discharged at roomtemperature or without being heated. Alternatively, an ink compositionmay be heated to a predetermined temperature to adjust the viscosity toan appropriate level and may be then discharged. Thus, good dischargestability is achieved.

The ink composition of the embodiment has a higher viscosity than thoseof aqueous ink compositions to be used in ordinary ink jet recordinginks. The viscosity therefore largely varies depending on changes intemperature when the ink composition is discharged. The change inviscosity of the ink highly affects the sizes of droplets and thedroplet-discharging speed, which may cause deterioration in imagequality. Accordingly, it is preferred to maintain the temperature of theink when it is discharged as constant as possible.

Curing Step

Subsequently, the ink composition discharged and adhering to therecording medium is cured by irradiation with light (ultraviolet light)in the curing step. This is caused by that the photopolymerizationinitiator contained in the ink composition is decomposed by ultravioletirradiation to generate initiation species such as radicals, acids, orbases and that polymerization of the polymerizable compound isaccelerated by the function of the initiation species, or is caused bythat polymerization of the polymerizable compound starts by ultravioletirradiation. If the ink composition contains a sensitizing dye togetherwith the photopolymerization initiator, the sensitizing dye in thesystem absorbs ultraviolet light and thereby changes to an excitedstate, which accelerates decomposition of the photopolymerizationinitiator by contact therewith. As a result, more sensitive curingreaction is achieved.

As the ultraviolet light source, a mercury lamp or a gas/solid laser ismainly used. As light sources for curing photocurable ink jet recordingink compositions, mercury lamps and metal halide lamps are widely used.From the viewpoint of environmental protection, however, it is currentlyhighly desired to be mercury-free, and replacement by GaN-basedsemiconductor ultraviolet light emitting devices is very usefulindustrially and environmentally. In addition, ultravioletlight-emitting diodes (UV-LEDs) and ultraviolet light-emitting laserdiodes (UV-LDs) are small-sized and have long life and high efficiencyand are low in cost, and are therefore expected as light sources forphotocurable ink jet recording. Among them, UV-LEDs are preferred.

It is preferable to use a photocurable ink composition that can be curedby irradiation using an LED preferably emitting light having an emissionpeak wavelength in a range of 350 to 420 nm, more preferably 390 to 400nm, at an irradiation energy of preferably 200 mJ/cm² or less, morepreferably 200 mJ/cm². In such a case, a high printing speed can beobtained at low cost. Such an ink composition can be obtained bycontaining a photopolymerization initiator that is decomposed byexposure to ultraviolet light having a wavelength in the above-mentionedrange and/or a polymerizable compound that starts polymerization byexposure to ultraviolet light having a wavelength in the above-mentionedrange.

Thus, according to the above-described two embodiments, it is possibleto provide a photocurable ink jet recording ink composition that hasexcellent curing property, color stability, and solubility of aphotopolymerization initiator and also has low viscosity and shows lessodor and skin irritation and provide a recording method using the inkcomposition. The photocurable ink jet recording ink composition of theembodiment can show desired effects, in particular, in photocuring byirradiation with ultraviolet light having an emission peak wavelength ina range of 350 to 420 nm.

EXAMPLES

The embodiments of the invention will now be more specifically describedby non-limiting examples.

Components

Components used in the following Examples and Comparative Examples areas follows.

Photopolymerization Initiator:

-   -   IRGACURE 819 (bisacylphosphine oxide type, trade name        manufactured by BASF, hereinafter abbreviated as “819”),    -   KAYACURE DETX-S (thioxanthone type, trade name manufactured by        Nippon Kayaku Co., Ltd., hereinafter abbreviated as “DETX-S”),        and    -   KAYACURE ITX (thioxanthone type, trade name manufactured by        Nippon Kayaku Co., Ltd., hereinafter abbreviated as “ITX”),        Polymerizable Compound:    -   VEEA (2-(2-vinyloxyethoxy)ethyl acrylate, trade name        manufactured by Nippon Shokubai Co., Ltd., hereinafter        abbreviated as “VEEA”),    -   Viscoat #192 (phenoxyethyl acrylate, trade name manufactured by        Osaka Organic Chemical Industry Ltd., hereinafter abbreviated as        “PEA”), and    -   APG-100 (dipropylene glycol diacrylate, trade name manufactured        by Shin-Nakamura Chemical Co., Ltd., hereinafter abbreviated as        “DPGDA”),        Pigment:    -   NC-006 (Color index name: C.I. Pigment blue 15:3, trade name        manufactured by Nikko Chem Tech Corporation, hereinafter        abbreviated as “15:3”),        Dispersant:    -   Solsperse 36000 (trade name manufactured by Lubrizol,        hereinafter abbreviated as “So136000”),        Leveling Agent:    -   Silicone surface conditioner BYK-UV3500 (trade name manufactured        by BYK, hereinafter abbreviated as “UV3500”), and        Polymerization Inhibitor:

MEHQ (hydroquinone monomethyl ether, trade name manufactured by KantoChemical Co., Inc., in Table 1, abbreviated as “MEHQ”).

Examples 1 to 12 and Comparative Examples 1 to 14

The components shown in Tables 1 to 3 were mixed in the amounts (unit: %by mass) shown in Tables 1 to 3, and the mixtures were stirred with ahigh-speed water-cooled stirrer to obtain cyan ink compositions.

In Tables 1 to 3, in the column “B/C”, “B” represents the amount ofinitiator B, i.e., bisacylphosphine oxide photopolymerization initiator(“819”), and “C” represents the amount of initiator C, i.e.,thioxanthone photopolymerization initiator (total of “DETX-S” and“ITX”). A numerical value of “B/C” being in the range of 1.25 or moreand 3 or less means that “B:C” is in the range of from 3:1 to 5:4.

TABLE 1 Trade Example Additive name 1 2 3 4 5 6 7 8 9 10 11 12 InitiatorB 819 6 6 6 6 5 5 5 6 6 6 6 6 Initiator C DETX-S 3 — 4 2 2 4 1.7 3 3 3 33 ITX — 3 — — — — — — — — — — Polymerizable VEEA 50 50 50 50 50 50 50 6070 55 20 45 compound PEA 25 25 25 25 25 25 25 15 10 20 35 30 DPGDA 9.99.9 8.9 10.9 11.9 9.9 12.2 9.9 4.9 9.9 29.9 9.9 Coloring 15:3 2 2 2 2 22 2 2 2 2 2 2 material Dispersant Sol36000 2 2 2 2 2 2 2 2 2 2 2 2Polymerization MEHQ 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6inhibitor Leveling agent UV3500 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.50.5 0.5 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0100.0 100.0 100.0 B/C 2 2 1.5 3 2.5 1.25 2.94 2 2 2 2 2

TABLE 2 Trade Comparative Example Additive name 1 2 3 4 5 6 7 InitiatorB 819 6 6 5 5 6 7 7 Initiator C DETX-S 5 1 1 5 — — — ITX — — — — — — —Polymerizable VEEA 50 50 50 50 50 50 — compound PEA 25 25 25 25 25 25 60DPGDA 7.9 11.9 12.9 8.9 12.9 11.9 26.9 Coloring material 15:3 2 2 2 2 22 2 Dispersant Sol36000 2 2 2 2 2 2 2 Polymerization MEHQ 1.6 1.6 1.61.6 1.6 1.6 1.6 inhibitor Leveling agent UV3500 0.5 0.5 0.5 0.5 0.5 0.50.5 Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 B/C 1.20 6 5 1 — — —

TABLE 3 Trade Comparative Example Additive name 8 9 10 11 12 13 14Initiator B 819 3 — — — 6 — 5.6 Initiator C DETX-S 3 6 12 12 3 12 1.4ITX — — — — — — — Poly- VEEA 50 50 50 52 10 — 50 merizable PEA 25 25 2525 50 50 25 compound DPGDA 12.9 12.9 6.9 4.9 24.9 31.9 11.9 Coloring15:3 2 2 2 2 2 2 2 material Dispersant Sol36000 2 2 2 2 2 2 2 Poly- MEHQ1.6 1.6 1.6 1.6 1.6 1.6 1.6 merization inhibitor Leveling UV3500 0.5 0.50.5 0.5 0.5 0.5 0.5 agent Total 100.0 100.0 100.0 100.0 100.0 100.0100.0 B/C 1 0 0 0 2 0 4

Evaluation Items

1. Solubility of Photopolymerization Initiator

Each mixture of components excluding the pigment was visually observedfor whether or not the photopolymerization initiators were completelydissolved in the process of dissolving the mixture by stirring (liquidtemperature: 20° C.). The evaluation results are shown in Tables 4 to 6.The evaluation criteria are as follows:

-   -   A: no undissolved photopolymerization initiators were observed        after the process of dissolving the mixture by stirring; and    -   B: undissolved photopolymerization initiators were observed        after the process of dissolving the mixture by stirring.        2. Viscosity

The viscosity of each ink composition prepared above was measured with arheometer MCR300 (manufactured by Paar Physica) under conditions of atemperature of 20° C. and a number of revolutions of 200 rpm. Theevaluation results are shown in Tables 4 to 6. The evaluation criteriaare as follows:

-   -   A: less than 11 mPa·s, and    -   B: 11 mPa·s or more.        3. Curing Property

The respective nozzle arrays of an ink jet printer, PX-G5000 (trade namemanufactured by Seiko Epson Corporation), were filled with thephotocurable ink compositions prepared above. A solid pattern image(recording resolution: 720×720 dpi) was printed on a PET film (Rumirer125E20 (trade name), manufactured by Panac Corporation) under ordinarytemperature and ordinary pressure with a dot diameter of middle so thatthe printed matter has a thickness of 7 μm, and the solid pattern imagewas cured by irradiation with 200 mJ/cm² of ultraviolet light having awavelength of 395 nm at an irradiation intensity of 1540 mW/cm² using aUV-LED in an ultraviolet irradiation device mounted on the side of acarriage. Thus, recorded matters of solid pattern images printed on PETfilms were produced. Here, the solid pattern image refers to an imageformed of dots recorded for every pixel, which is a minimum recordingunit region defined by recoding resolution.

The irradiation energy [mJ/cm²] was determined as the product obtainedby multiplying the irradiation intensity [mW/cm²] on the surfaceirradiated with light from a light source by the irradiation durationtime [s]. The irradiation intensity was measured using an ultravioletray intensity meter, UM-10, and a receptor head, UM-400 (bothmanufactured by Konica Minolta Sensing, Inc.).

Whether a coating film was in a tack-free state or not was determinedbased on the following conditions: whether an ink adhered to a cottonswab or not or whether a cured ink material on a recording medium wasabraded or not. The cotton swab used was Johnson cotton swabmanufactured by Johnson & Johnson. Each coating film was rubbed with thecotton swab back and forth 10 times under a load of 100 g.

The ink coating film (cured film) used for the curing propertyevaluation had a thickness of 7 μm. The evaluation results are shown inTables 4 to 6, and the evaluation criteria are as follows:

-   -   A: no tack was observed on the surface, and    -   B: tack was observed on the surface.        4. Color Stability

The colors of the recorded matters prepared above were measured usingthe CIE Lab (L*a*b* color system). Specifically, the recorded matterswere left to stand for seven days after recording and were measured forcolors. The hue at this time was compared with that at 2 hours after therecording, and the difference (ΔE) in the hue was determined by thefollowing expression:ΔE=(Δa*^2+Δb*^2+ΔL*^2)^(1/2).

The evaluation results are shown in Tables 4 to 6, and the evaluationcriteria are as follows:

-   -   A: ΔE was less than 1.5, and    -   B: ΔE was 1.5 or more.

TABLE 4 Example Evaluation item 1 2 3 4 5 6 7 8 9 10 11 12 Initiator A AA A A A A A A A A A solubility Viscosity A A A A A A A A A A B B CuringA A A A A A A A A A A A property Color stability A A A A A A A A A A A A

TABLE 5 Comparative Example Evaluation item 1 2 3 4 5 6 7 Initiatorsolubility B B B B B B A Viscosity A A A A A A B Curing property A A B AB A B Color stability B A A B A A A

TABLE 6 Comparative Example Evaluation item 8 9 10 11 12 13 14 Initiatorsolubility A A B B A A B Viscosity A A A A B A A Curing property B B B BB B A Color stability A B B B A B A

The results shown in Tables 4 to 6 reveal that the ink compositions(Examples 1 to 12) each containing a polymerizable compound and aphotopolymerization initiator, where the polymerizable compound includes50% by mass or more of vinyl ether group-containing (meth)acrylic estersrepresented by Formula (I), and the photopolymerization initiatorincludes a bisacylphosphine oxide type in an amount of 5% by mass ormore and a thioxanthone type at a content ratio of from 3:1 to 5:4, havefurther excellent curing property and solubility of thephotopolymerization initiator and equivalent or higher color stabilitycompared with those of other ink compositions (Comparative Examples 1 to14).

Furthermore, when the value of B/C in the item of “initiator solubility”is less than 1.25 or higher than 3 (which is out of the range of thecontent ratio of initiator B to initiator C of from 3:1 to 5:4), thephotopolymerization initiator is not sufficiently dissolved.

The entire disclosure of Japanese Patent ApplicationNo.2011-198184,filed Sep. 12,2011 IS expressly incorporated by referenceherein.

What is claimed is:
 1. A photocurable ink jet recording ink compositioncomprising a polymerizable Compound and a photopolymerization initiator,wherein the polymerizable compound includes: vinyl ethergroup-containing (meth)acrylic esters represented by the followingFormula (1):CH₂═CR¹—COOR²—O—CH═CH—R³  (I) (wherein, R1 represents a hydrogen atom ora methyl group; R2 represents a divalent organic residue having 2 to 20carbon atoms; and R3 represents a hydrogen atom or a monovalent organicresidue having 1 to 11 carbon atoms) in an amount of 20% by mass or morebased on the total mass of the ink composition; the photopolymerizationinitiator includes a bisacylphosphine oxide photopolymerizationinitiator in an amount of 5% by mass or more based on the total mass ofthe ink composition and a thioxanthone photopolymerization initiator,wherein the content ratio of the bisacylphosphine oxidephotopolymerization initiator to the thioxanthone photopolymerizationinitiator is from 3:1 to 5:4 by mass standard; and the content of thevinyl ether group-containing (meth)acrylic esters is 50% by mass ormore, or the total content of polymerizable compound other than thevinyl ether group containing (meth)acrylic esters is 64.9% by mass orless, wherein the polymerizable compound further includes amonofucntional (meth)acrylate other then the vinyl ethergroup-containing (meth)acrylic esters represented by formula (1), andwherein the content of the monofunctional acrylate other then the vinylether group-containing (meth)acrylic esters represented by formula (1)is 20% by mass based on the total mass of the ink composition.
 2. Thephotocurable ink jet recording ink composition according to claim 1,wherein the thioxanthone photopolymerization initiator is at least oneof 2,4-diethylthioxanthone and 2- isopropylthioxanthone.
 3. Thephotocurable ink jet recording ink composition according to claim 1,wherein the content of the thioxanthone photopolymerization initiator is4% by mass or less based on the total mass of the ink composition. 4.The photocurable ink jet recording ink composition according to claim 1,wherein the bisacylphosphine oxide photopolymerization initiator isbis(2,4,6 trimethylbenzoyl)phenylphosphine oxide.
 5. The photocurableink jet recording ink composition according to claim 1, wherein thepolymerizable compound further includes phenoxyethyl (meth)acrylate inan amount of less than 30% by mass based on the total mass of the inkcomposition.
 6. The photocurable ink jet recording ink compositionaccording to claim 1, wherein the vinyl ether group-containing(meth)acrylic esters are 2-(vinyloxyethoxy)ethyl acrylate.
 7. Thephotocurable inkjet recording ink composition according to claim 1,wherein the polymerizable compound further includes amonofunctional(meth) acrylate having an aromatic ring skeleton.
 8. Anink jet recording method comprising: discharging a photocurable ink jetrecording ink composition according to claim 1 onto a recording medium;and curing the discharged ink composition by irradiating the inkcomposition with ultraviolet light having an emission peak wavelength ina range of 350 to 420 nm using an LED.
 9. An ink jet recording methodcomprising: discharging a photocurable ink jet recording ink compositionaccording to claim 2 onto a recording medium; and curing the dischargedink composition by irradiating the ink composition with ultravioletlight having an emission peak wavelength in a range of 350 to 420 nmusing an LED.
 10. An ink jet recording method comprising: discharging aphotocurable ink jet recording ink composition according to claim 3 ontoa recording medium; and curing the discharged ink composition byirradiating the ink composition with ultraviolet light having anemission peak wavelength in a range of 350 to 420 nm using an LED. 11.An ink jet recording method comprising: discharging a photocurable inkjet recording ink composition according to claim 4 onto a recordingmedium; and curing the discharged ink composition by irradiating the inkcomposition with ultraviolet light having an emission peak wavelength ina range of 350 to 420 nm using an LED.
 12. An ink jet recording methodcomprising: discharging a photocurable ink jet recording ink compositionaccording to claim 5 onto a recording medium; and curing the dischargedink composition by irradiating the ink composition with ultravioletlight having an emission peak wavelength in a range of 350 to 420 nmusing an LED.
 13. An ink jet recording method comprising: discharging aphotocurable ink jet recording ink composition according to claim 6 ontoa recording medium; and curing the discharged ink composition byirradiating the ink composition with ultraviolet light having anemission peak wavelength in a range of 350 to 420 nm using an LED. 14.An ink jet recording method comprising: discharging a photocurable inkjet recording ink composition according to claim 7 onto a recordingmedium; and curing the discharged ink composition by irradiating the inkcomposition with ultraviolet light having an emission peak wavelength ina range of 350 to 420 nm using an LED.